Chillers operate on natural gas or propane gas. See Fig. 17-2. Gas is used for the major job of cooling. Electricity is used for the smaller energy requirements of fans, motors, and controls. This means electrical power requirements are only about 20 percent of those of a completely electrical unit.
Gas units are available in 3-, 4-, and 5-ton capacities. They use heat as a catalyst. They have no compressor. This means they have fewer moving parts than other types of systems. They are designed for outside installation. They cool by circulating, the refrigerant, which is plain tap water, through a matching coil. The coil is added to a new or existing furnace in the house. As the chilled water produced by the unit circulates through the coil, it absorbs heat from the conditioned space. The water, bearing the absorbed heat, is then returned to the unit outdoors, where the heat is dissipated to the outside air. Table 17-1 shows the amount of permanent antifreeze required when the outside temperature is below freezing. A defoaming agent also must be added.
Table 17-2 shows the specifications of Arkla 3-, 4-, and 5-ton units. Note that the refrigerant is R-717 (ammonia). Also, note the amount of gas needed to produce 3 tons of air conditioning—79,000 Btu.
Gas-fired units may be connected in 5-ton multiples to provide up to 30 tons of air conditioning. Figure 17-3 shows how they are doubled up to provide 10 tons. For some units, it takes 250,000 Btu of gas input per hour to provide 120,000 Btu/h of cooling. That means 48 percent efficiency, if the electrical energy needed is not accounted for in the figuring. For the unit referred to, the operating voltage is 230 V, with 60-Hz, single-phase operation. Wiring size is not too large, since there is a maximum of 8 A drawn for the condenser fan motors and 33 A for the solution pumps. Normal running current for the solution pump motors is only 5 A. Locked rotor current of 33 A occurs only if the motor is stuck or jammed so it cannot start. The start currents may also reach this 33-A level under some load conditions.
The chilled water system uses stainless steel to prevent problems with rust and other ferrous metal piping problems. There is only one electrical, one gas, and one chilled water supply and return connection for each unit.